This invention generally relates to a process for producing agglomerates of metal powders. More particularly, this invention is directed to a process for producing rigid, porous, binder free agglomerates of metal powders. This invention is also directed to devices that include the rigid, binder free agglomerates.
Fine metal powders are used in a wide variety of devices to enable desirable chemical reactions. For example, catalysts are incorporated in the catalytic converts of vehicles powered by combustion engines. The catalyst facilitates the conversion of potentially harmful fumes to environmentally acceptable gases or liquids. In another example, metal powders are used to store gases, such as hydrogen, in a solid matrix to minimize the hazards associated with the storage and transport of hydrogen as a compressed gas. Fine metal powders are also used in batteries and fuel cells. Commercially available batteries, including both rechargeable and non-rechargeable batteries, are used to power portable devices such as flashlights, cameras and tape recorders. One chemical system used to produce rechargeable batteries incorporates a finely divided metal hydride in one of the electrodes. Another chemical system, typically used to manufacture non-rechargeable batteries, also known as primary batteries, uses an alkaline electrolyte, manganese dioxide as the active cathode material and zinc as the active anode material. The zinc is usually disposed within the central region of the battery as part of a gel. Prior to incorporating the zinc into the battery, the zinc is comminuted so that a quantity of zinc powder with a majority of particles ranging from 25 microns to 500 microns is obtained. The individual particles are suspended in the anode gel which prevents settling of the zinc particles within the battery.
One of the long-standing objectives of battery manufacturers is to produce batteries with the ability to power a device for longer and longer periods of time. The need to improve the battery's performance is especially acute in devices that require large currents. As disclosed in JP Kokai 57[1982]-182972, the high discharge characteristic of a battery can be improved by incorporating 5-30 weight percent of the zinc as particles with a particle size of 25 microns or smaller. Unfortunately, as the percentage of particles that are 25 microns or smaller increases, the viscosity of the anode may become too high to process in high speed manufacturing machines. One way to overcome this problem is to process all of the zinc particles into a single porous body. For example, U.S. Pat. No. 2,480,839 discloses an anode made of zinc powder or particles that have been compressed under sufficient pressure to agglomerate the particles into a coherent body shaped as a hollow cylinder. In another example, U.S. Pat. No. 3,645,793 describes cleaning the zinc powder with a mild acid and then pressing the zinc to form a porous structure. These patents are directed to the production of coherent structures that are suitable for use as an electrode in an electrochemical cell. All of the particles are included in the compaction process and form a part of the compacted electrode. Thus, these processes are not well suited for the production of electrodes that incorporate both agglomerated electrochemically active particles and non-agglomerated electrochemically active particles in the same electrode.
Other methods of handling the finely divided metal powders include the step of utilizing an agglomerant to form the agglomerates. The agglomerant may be a binder that acts as an adhesive to secure particles to one another thereby enabling the formation of the agglomerates. Alternatively, the agglomerant may be a pore former which facilitates the formation of the agglomerate but is then removed from the agglomerate thereby forming pores within the agglomerate. Unfortunately, the use of an agglomerant may have a negative impact on the performance of the agglomerated powder. For example, if a battery includes an electrode that uses agglomerates of electrochemically active material that incorporate an organic binder, such as polyvinyl alcohol (PVA), then the particles are inherently coated with the electrically nonconductive PVA. The coating increases the internal resistance of the electrode that includes the coated, agglomerated particles. As the electrode's internal resistance increases, the battery's run time decreases. Furthermore, there are potential problems associated with the cost of the binder as well as the volume of space occupied by the binder. As the volume of space dedicated to the binder increases, the quantity of electrochemically active material must be decreased to make room for the binder. As the quantity of active material is decreased, the cell's run time is reduced.
Therefore, there exists a need for a process that produces small, rigid, binder free agglomerates that do not compromise the performance of the agglomerated particles. The process should not require the use of an additive, such as a binder or pore former, to enable production of the agglomerates.